FB1: How to select an electrical power system: motor and battery

All current (mid 2013) Nerf flywheel blasters use 130-sized motors with a 25 mm length (there are 130-sized motors that are longer). A popular replacement for these stock motors is the Solarbotics RM2 motors. Additional alternatives include all of the Tamiya Mini series motors. Although the Tamiyas run at considerably lower voltage (3V) comparing to the RM2s, their ultra high Kv more than makes up for it. Tamiyas are to a Ferrari as the RM2s are to a Lamborghini. One is finely engineered while the other is purely brutal. At this point, choosing between the two types is mostly based on preference. In terms of the performance, a range test was conducted between the two motors and the results are inconclusive. Once the chronometer is built (currently under construction), a comparison will be made again and the results will be posted. The Tamiyas might be a bit more expensive, but there are also built with higher quality, making them more reliable and consistent. Battery availability should also be considered when choosing between the motors.
Although the motors can be chosen indifferently, the batteries cannot. The motor and battery combination must be carefully matched to optimize the system. Since there are various types and vast numbers of different batteries out there, but only a few choices of motors, it is easier to first select the motor as the limiting factor, and then match the battery to the selected motor. For the most part, there are only two key specifications of a battery that needs to be considered: output voltage and maximum burst current. In terms of the output voltage, simply match it to, or slightly higher above (over-volting), the specified maximum operating voltage of the motor. There is a small tricky part to this step. The advertised voltage of a battery is usually not the voltage it outputs at full charge. For example, the advertised voltage of a rechargeable NiMH AA battery may be 1.2 V, but it can output as high as 1.46 V when charged. Similarly, the advertised voltage for a LiPo cell is 3.7 V, but it outputs 4.2 V when charged. The reason for this is because the advertised voltage is usually the nominal, storage and/or some other defined voltage parameter of the battery. However, this does not mean that it is their highest capable output voltage. Another important parameter to consider is the output voltage of the battery as a function of time during use. Normal Alkaline batteries' voltage decreases linearly with time. NiMH rechargeable batteries' voltage decreases in an inverse exponential form, allowing them to maintain their maximum output voltage for longer.

Fig. FB1.1 – voltage vs. time comparison

Although it is encouraged to develop a system based on preference, please make sure to choose parts that match the desired specifications to create a high performance, durable system.
A common modification conducted by the Nerf community is to use unprotected TrustFire 14500 batteries in their systems. Although this modification is simple, clean and cheap, engineering-wise, it is a terrible design and highly not recommended. The TrustFire 14500 has a maximum continuous discharge current of 2.5 C, which means that the maximum burst output current is approximately 4.5 A. The stall current of a RM2 motor at 12.6 V is around 12.42 A (probably around 6.21 A for a flywheel system). With two RM2 motors for a flywheel system, the required burst current is 24.84 A (or 12.42 A for a flywheel system), which is no where near what the TrustFire batteries are capable of producing. Evidence of this can be physically observed by the slow speedup time of a RM2-TrustFire system. Currently, LiPo batteries are the best type of batteries to use in a hobby-level, high power electrical system. The new generation of LiPo batteries are very safe and reliable, provided that they are used correctly. They are capable of producing immense power, able to contain high capacity, relatively light, widely available and cheap. They also have a high number of recharge cycles for its lifetime. The downsides for using LiPo batteries are that it requires a special charger which costs around $40, and also it may be dangerous when handled by a novice. To create the best system possible, LiPo batteries are an absolute necessity because of their low internal resistance. If moderate modifications are desired, other battery types will do the job so long as the specifications are met. Instead of using TrustFire batteries, RC-grade NiMH packs are a good alternative. They are usually rated for anywhere from 10 to 20 A of continuous current and are extremely safe, with almost no possibility of catching fire in any condition.
Below are a few useful battery equations:

Equation FB1.1 – continuous current equation

Note, Cn is the normalization constant for the discharge rate calculation. Specifications for a battery usually lists Icont as the following:
Discharge rate: 25C
This implies that the continues discharge rate (Icont) is equal to 25 times the capacity of the battery in Ah. In this case, 25 is the Cn. Icont should be greater than the continuous current of the motor(s) under light load (powering flywheels).

Equation FB1.2 – max current approximation

This approximation is valid for most types of batteries such as LiPo batteries. Imax should be greater than the stall current of the motor(s).

Equation FB1.3 – battery run time approximation in minutes

A few good sources for motor and battery purchase are listed below:
  • Hobbylinc: http://www.hobbylinc.com/
    Comments: Stocks almost every Tamiya Mini series motors with a good discount. Fast shipping and handling (~1 week package arrival).
  • TamiyaUSA: http://www.tamiyausa.com/
    Comments: MSRP pricing on Tamiya motors, but good stock availability (in case Hobbylinc is out). Medium shipping and handling speed (<3 weeks package arrival)
  • Pololu: http://www.pololu.com/
    Comments: Good source for RM2s and other industrial-type 130-sized motors. Fast shipping and handling (~1 week package arrival).
  • Tower Hobbies: http://www.towerhobbies.com/
    Comments: Great source for many RC batteries. Also stocks some Tamiya motors. Handling is fast but the value shipping method is slow (~2 weeks package arrival with value shipping).
  • Hobbypartz: http://www.hobbypartz.com/
    Comments: Huge selection of high quality as well as cheap LiPo batteries (high quality Chinese LiPos). Availability is somewhat low (they sell out really fast after receiving stock). Good source for a LiPo charger as well. Fast shipping and handling (~1 week package arrival).
  • Hobbyking: http://www.hobbyking.com/hobbyking/store/index.asp
    Comments: Huge selection and availability of RC batteries including decent quality LiPos (average quality Chinese LiPos). Shipping and handling is fast for the USA warehouse products (<2 weeks package arrival), slow for the international warehouse products (3 weeks to 2 months package arrival).


  1. I must comment that RM2s are notoriously unreliable. Also, if you use them with a proper lipo (2S or more) they will die absurdly quickly. So people ought to use trustfires with them actually or they'll just die cause those brushes are so bad.

    1. Actually 2s is within specs. More than that will cause brush failures. If you read our Rayven guide, we address this under the Parts section.

    2. Azrael, I followed your work on motors on NH and was very impressed. We at the Britnerf forum have a member who has developed a carbon brush version of the RM2. I will follow this blog with interest, thanks for posting.

    3. Oh cool. Yes, that's a great mod for the RM2s, I actually have some parts on the way for some similar modifications. Stay tuned, I have some new ideas to try out.

      This article in particular was written by my partner, infamous, just so we know. He was too lazy to accept admin rights at the time of this posting hahah.

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  2. As a noob, what are the model numbers on the Tamiya motors?

    1. We used the Mach 2 Pro Motors in our build. :)

  3. If using a non-flywheel system, like the old Stampede, or Swarmfire, would you still say that the Trustfires are no good? I am looking for something like the ease of the trustfires so I didn't have to mod the battery compartment.. Is there any better alternative or other batteries that you would recommend, or always LiPo's?

    1. I have had good luck with using eFest 14500 sized IMRs in my Swarmfire. These have 4x the discharge rate of Trustfires. With some nested springs, I was firing around 105 fps average, I think. This is with 12.6v, or a 3s system.

      For a Stampede, they are a bit more current hungry, I think. I use a 4s LiPo (16.8v) in mine. However, you may find that as you increase voltage, more modification is necessary to make the blaster function reliably. I had to increase the spring load greatly to prevent the bolt from returning too quickly, and then also modify the shape of some of the internals to prevent runaway firing. Using around 11kg of spring force, I was shooting around 95-100 fps, I think. I do not think Trustfires are a good idea here, due to the increased current consumption. SGNerf got away with it, but my rate of fire is much higher than his, and my blaster fires more consistently. He also admitted that some of his builds did get runaway firing.

  4. This comment has been removed by the author.

  5. if i wanna mod my rapidstrike. what motor and batteries should i use?

    1. Really hard to say - there's a lot of different camps based around different sets of motors. But as long as you use batteries with a good rate of current discharge(whether you go 18650, LiPo, or NiMH), you will be fine. Try and figure out how much stall current your motors will ask for.

  6. Hi,
    Awesome post and very informative. I had a question about current and how it affects motors. If I understand your post correctly, the current a battery can provide is defined by C*Cn. For example, I have a 3S 4000mAh 20C LiPo, meaning the Icont would be 80 A. Does this mean the battery is constantly sending 80 A of current through the circuit, and if so, would this be bad for the motor?


    1. What you have worked out is what your battery is capable of. However, what it will actually deliver depends on what your motor will ask for. It will only provide as much power as your motor needs. Does that make sense?